Hitherto, a magnetic recording medium composed of a nonagnetic support having formed thereon a magnetic layer comprising a ferromagnetic powder composed of the acicular crystals of .gamma.-Fe.sub.2 O.sub.3, CrO.sub.2, Co-doped Fe.sub.2 O.sub.3, etc., dispersed in a binder has been widely used for magnetic recording and reproduction.
However, recently, there has been a strong demand to improve recording density to attain a large capacity for recording and to miniaturize a magnetic recording device. But in order to obtain a magnetic recording medium suitable for high-density recording using a conventional acicular ferromagnetic powder, it is necessary to sufficiently reduce the maximum size of the acicular ferromagnetic powder than the recording wavelength or the recording bit length. At present, an acicular ferromagnetic powder having a size of about 0.3 .mu.m has been practically used and a recording wavelength of as low as about 1 .mu.m has been obtained.
To obtain a magnetic recording medium capable of far higher density recording, it is necessary to further reduce the size of the acicular ferromagnetic powder. However, with such small acicular ferromagnetic powder, since the diameter thereof is as thin as below 100 .ANG. and the volume of the particle is as small as below 10.sup.-17 cm.sup.3, there is the problem that the magnetic characteristics are lowered by the effects of a thermal disturbance on the surface. Also, when a magnetic field is applied to the coated magnetic layer, a sufficient orientation of the ferromagnetic powder is not obtained.
Recently, to solve these problems, a magnetic recording medium using a hexagonal ferrite ferromagnetic substance having a tabular form and an axis of easy magnetization in the direction perpendicular to the plane was developed as described, e.g., in JP-A-58-6525 and JP-A-58-6526 (the term "JP-A" as used herein means an "unexamined published Japanese patent application"). By this development, it is possible to reduce the mean particle size (diameter) of the ferromagnetic powder below 0.05 .mu.m and high density recording becomes possible.
Also, for improving the abrasion resistance of the magnetic layer of a magnetic recording medium and a magnetic head, the present inventor previously proposed that abrasives composed of Al.sub.2 O.sub.3 having particle sizes of from 0.003 to 0.1 .mu.m and a nonmagnetic powder having a Mohs' hardness of at least 5 and particle sizes of from 0.3 to 2.0 .mu.m be added to the magnetic layer of a magnetic recording medium as disclosed in JP-A-58-56227. The present inventor also proposed that in a magnetic recording medium having a back layer on the opposite side of the support to the side carrying a magnetic layer, inorganic powders composed of a mixture of fine inorganic particles having a mean particle size of from 0.01 to 0.1 .mu.m and coarse inorganic particles having a mean particle size of from 0.1 to 0.8 .mu.m be added to the back layer, whereby the S/N ratio, the friction coefficient, and the running durability of the magnetic recording medium could be improved as described in JP-A-57-53825.
Since hexagonal ferrite fine powder has a low saturation magnetization compared to that of Co-doped Fe.sub.2 O.sub.3 ferromagnetic powder, a metal (or alloy) ferromagnetic powder, etc., and hence a high output is difficult to obtain when using a hexagonal ferrite fine powder, the packing density of the hexagonal ferrite fine powder must be increased to provide a magnetic recording medium giving a high output. However, since the hexagonal ferrite fine powder has a fine particle size and the form thereof is hexagonal, the dispersibility thereof in a binder is inferior to conventional ferromagnetic powders. Hence it is fundamentally difficult to insure the running durability of the magnetic recording medium by keeping a low friction coefficient. Thus, in order to insure the running durability of a magnetic recording medium using a hexagonal ferrite fine powder, abrasives composed of fine inorganic particles and coarse inorganic particles have to be used. Also the amount of the abrasives and hence the amount of the binder resin must be increased, thereby causing the problem that the packing density of the hexagonal ferrite fine powder must be sacrificed. It is therefore difficult to simultaneously achieve reproducing output and running durability.
Also, even when one intends to improve reproducing output by increasing the saturation magnetization amount, in the case of using a hexagonal ferrite fine powder having a large surface area, the dispersibility thereof is reduced. Consequently, it is difficult to simultaneously improve reproducing output and running durability of the magnetic recording medium.